Nanowire Interface Research Articles

W18O49 nanowires grown on g-C3N4 sheets with enhanced photocatalytic hydrogen evolution activity under visible light

A hybrid nanomaterial comprising of one-dimensional W18O49 nanowires grown on two-dimensional g-C3N4 sheets was prepared by a facile solvothermal process. Photocatalytic activity of these materials for H2 generation under visible light was evaluated. The optimum efficiency of W18O49/g-C3N4 composite photocatalyst at a mass content of 30% was up to 4.5 times as high as that of the pure g-C3N4. The significantly enhanced performance of W18O49/g-C3N4 composites was mainly attributed to the synergistic effect between the interface of W18O49 nanowires and g-C3N4 sheets, involving stronger light absorption in the visible range, faster transfer and more efficient separation of the photo-generated electron-hole pairs. In short, the W18O49/g-C3N4 hybrid nanomaterial was demonstrated to be an effective photocatalyst for H2 production.

Sandwiched assembly of ZnO nanowires between graphene layers for a self-powered and fast responsive ultraviolet photodetector

A heterostructure of graphene and zinc oxide (ZnO) nanowires (NWs) is fabricated by sandwiching an array of ZnO NWs between two graphene layers for an ultraviolet (UV) photodetector. This unique structure allows NWs to be in direct contact with the graphene layers, minimizing the effect of the substrate or metal electrodes. In this device, graphene layers act as highly conducting electrodes with a high mobility of the generated charge carriers. An excellent sensitivity is demonstrated towards UV illumination, with a reversible photoresponse even for a short period of UV illumination. Response and recovery times of a few milliseconds demonstrated a much faster photoresponse than most of the conventional ZnO nanostructure-based photodetectors. It is shown that the generation of a built-in electric field between the interface of graphene and ZnO NWs effectively contributes to the separation of photogenerated electron–hole pairs for photocurrent generation without applying any external bias. Upon application of external bias voltage, the electric field further increases the drift velocity of photogenerated electrons by reducing the charge recombination rates, and results in an enhancement of the photocurrent. Therefore, the graphene-based heterostructure (G/ZnO NW/G) opens avenues to constructing a novel heterostructure with a combination of two functionally dissimilar materials.

High-Performance Fully Nanostructured Photodetector with Single-Crystalline CdS Nanotubes as Active Layer and Very Long Ag Nanowires as Transparent Electrodes.

Long and single-crystalline CdS nanotubes (NTs) have been prepared via a physical evaporation process. A metal-semiconductor-metal full-nanostructured photodetector with CdS NTs as active layer and Ag nanowires (NWs) of low resistivity and high transmissivity as electrodes has been fabricated and characterized. The CdS NTs-based photodetectors exhibit high performance, such as lowest dark currents (0.19 nA) and high photoresponse ratio (Ilight/Idark ≈ 4016) (among CdS nanostructure network photodetectors and NTs netwok photodetectors reported so far) and very low operation voltages (0.5 V). The photoconduction mechanism, including the formation of a Schottky barrier at the interface of Ag NW and CdS NTs and the effect of oxygen adsorption process on the Schottky barrier has also been provided in detail based on the studies of CdS NTs photodetector in air and vacuum. Furthermore, CdS NTs photodetector exhibits an enhanced photosensitivity as compared with CdS NWs photodetector. The enhancement in performance is dependent on the larger surface area of NTs adsorbing more oxygen in air and the microcavity structure of NTs with higher light absorption efficiency and external quantum efficiency. It is believed that CdS NTs can potentially be useful in the designs of 1D CdS-based optoelectronic devices and solar cells.

Flexible piezoelectric nanogenerators based on silicon nanowire/α-quartz composites for mechanical energy harvesting

The α-quartz could grow at the interface of silicon nanowires (SiNWs) after thermal treatment. The SiNW/α-quartz composite was utilized as an active layer to fabricate piezoelectric nanogenerators on a flexible polyethylene terephthalate substrate, capable of converting mechanical energy into electricity. An insulative Al2O3 layer was designed to fill the gaps of the composite to avoid the leakage of current by atomic layer deposition and reactive ion etching process. The flexible nanogenerators produced an average short-circuit current of 0.6μA under the force of 60N and the maximum power value reached about 380nW with the load resistance of 20MΩ.

Ultrafast Carrier Dynamics of a Photo-Excited Germanium Nanowire–Air Metamaterial

Ultrafast carrier dynamics in arrays of single crystal and relatively uniform-diameter Ge nanowires (NWs) are investigated by transient absorption measurements and effective medium simulations. We present the first quantitative analysis of a Ge NW–air metamaterial, translating the photon response of the assemblies to carrier dynamics. Three time regimes of the ultrafast recombination process are identified: Auger recombination dominant (0–5 ps), “fast” surface trapping and recombination dominant (5–20 ps), and a mix of “fast” recombination and “slow” surface trapping (20–200 ps). The rates of surface recombination and their dependences on pump fluence are determined, highlighting the different interactions of electrons and holes with Ge NW surface and interface states. Structural and excitation conditions can be engineered to extend the photogenerated electron and hole lifetimes. Small wire diameters and low pump powers enhance the electron lifetime because charging of defect states in the surface oxide l...

Improving Performance via Blocking Layers in Dye-Sensitized Solar Cells Based on Nanowire Photoanodes.

Electron recombination in dye-sensitized solar cells (DSSCs) results in significant electron loss and performance degradation. However, the reduction of electron recombination via blocking layers in nanowire-based DSSCs has rarely been investigated. In this study, HfO2 or TiO2 blocking layers are deposited on nanowire surfaces via atomic layer deposition (ALD) to reduce electron recombination in nanowire-based DSSCs. The control cell consisting of ITO nanowires coated with a porous shell of TiO2 by TiCl4 treatment yields an efficiency of 2.82%. The efficiency increases dramatically to 5.38% upon the insertion of a 1.3 nm TiO2 compact layer between the nanowire surface and porous TiO2 shell. This efficiency enhancement implies that porous sol-gel coatings on nanowires (e.g., via TiCl4 treatment) result in significant electron recombination in nanowire-based DSSCs, while compact coatings formed by ALD are more advantageous because of their ability to act as a blocking layer. By comparing nanowire-based DSSCs with their nanoparticle-based counterparts, we find that the nanowire-based DSSCs suffer more severe electron recombination from ITO due to the much higher surface area exposed to the electrolyte. While the insertion of a high band gap compact layer of HfO2 between the interface of the conductive nanowire and TiO2 shell improves performance, a comparison of the cell performance between TiO2 and HfO2 compact layers indicates that charge collection is suppressed by the difference in energy states. Consequently, the use of high band gap materials at the interface of conductive nanowires and TiO2 is not recommended.

Palladium-Gold Alloy Nanowire-Structured Interface for Hydrogen Sensing.

Understanding how the detailed nanoscale structure governs the interfacial interactions and reactivities is key to the exploration of nanostructured chemical sensors. We describe herein novel findings of an investigation of a palladium-gold alloy nanowire interface for hydrogen sensing. A dielectrophoretic growth pathway was utilized for controllable fabrication of the alloyed nanowires with minimum branching structures on a microelectrode device using controlled ratios of palladium and gold precursors in a two-step mechanism, a nucleation process initiated at a lower alternating current (AC) frequency followed by a growth process at a higher frequency up to 15 MHz. The nanowires showed a reduced branching propensity and highly oriented 1D feature, with the bimetallic composition scaling linearly with the frequency. The nanowires exhibited excellent responses to hydrogen in concentrations as low as 0.5 % by volume. The hydrogen-response characteristic represents an optimized balance of the gold-induced lattice expansion of palladium and hydrogen adsorption-induced phase and stress changes, a new insight into the sensing mechanism of the alloy nanowire. The mechanistic sensing details are also discussed in correlation with the growth mechanism, which provides a new insight into the synergy of the bimetallic composition of the alloy nanowires for the enhanced sensitivity for the detection of hydrogen.

The Current Crowding Effect in ZnO Nanowires with a Metal Contact

Nanoscale electron tunneling and thermionic transport via the metal-semiconductor interface of ZnO nanowires is found to be strongly affected by the metal contact shape, the metal-semiconductor interface radius and the nanowire radius. Our full 3D simulations show that the current crowding effect occurs for end-bonded contacts due to the geometry variation in the case of Schottky Au-ZnO nanowire interfaces. The change in the geometry will also lead to a change in the electrical behavior of the contact from Schottky to Ohmic which can substantially reduce the contact resistance.

Interaction between lamellar twinning and catalyst dynamics in spontaneous core-shell InGaP nanowires.

Semiconductor nanowires oriented along the [211] direction usually present twins parallel to their axis. For group IV nanowires this kind of twin allows the formation of a catalyst-nanowire interface composed of two equivalent {111} facets. For III-V nanowires, however, the twin will generate two facets with different polarities. In order to keep the <211> orientation stable, a balance in growth rates for these different facets must be reached. We report here the observation of stable, micron-long <211>-oriented InGaP nanowires with a spontaneous core-shell structure. We show that stacking fault formation in the crystal region corresponding to the {111}A facet termination provides a stable NW/NP interface for growth along the <211> direction. During sample cool down, however, the catalyst migrates to a lateral {111}B facet, allowing the growth of branches perpendicular to the initial orientation. In addition to that, we show that the core-shell structure is non-concentric, most likely due to the asymmetry between the facets formed in the NW sidewall; this effect generates stress along the nanowire, which can be relieved through bending.

Selective-process-based silicon nanowires batch fabrication technique and its surface biomolecule assembly with the sensor applications

Aiming at the manufacturing demand of silicon nanowires in detection application in biological, medical, environmental and anti-terroristic area, silicon nanowire fabrication technique was developed using traditional lithography and etching process on silicon wafer. The well-aligned assemble method of multiple biological probes (including protein probe and gene probe) on silicon nanowire interface was studied carefully. The top-down batch manufacturing method of silicon nanowire with good consistency and precisely controlled technology of biological species recognization and acquisition were proposed, laying solid technical foundation for the production of high sensitive and high selective silicon nanowire biosensor and providing new principle and method for silicon nanowire and related biological assembly.

Giant magnetoconductance oscillations in hybrid superconductor-semiconductor core/shell nanowire devices.

The magnetotransport of GaAs/InAs core/shell nanowires contacted by two superconducting Nb electrodes is investigated, where the InAs shell forms a tube-like conductive channel around the highly resistive GaAs core. By applying a magnetic field along the nanowire axis, regular magnetoconductance oscillations with an amplitude in the order of e(2)/h are observed. The oscillation amplitude is found to be larger by 2 orders of magnitude compared to the measurements of a reference sample with normal metal contacts. For the Nb-contacted core/shell nanowire the oscillation period corresponds to half a flux quantum Φ0/2 = h/2e in contrast to the period of Φ0 of the reference sample. The strongly enhanced magnetoconductance oscillations are explained by phase-coherent resonant Andreev reflections at the Nb-core/shell nanowire interface.

Enhanced laminin adsorption on nanowires compared to flat surfaces

Semiconductor nanowires are widely used to interface living cells, and numerous nanowire-based devices have been developed to manipulate or sense cell behavior. We have, however, little knowledge on the nature of the cell–nanowire interface. Laminin is an extracellular matrix protein promoting cell attachment and growth. Here, we used a method based on fluorescence microscopy and measured the relative amount of laminin adsorbed on nanowires compared to flat surfaces. The amount of adsorbed laminin per surface area is up to 4 times higher on 55nm diameter gallium phosphide nanowires compared to the flat gallium phosphide surface between the nanowires. We show that this enhanced adsorption on nanowires cannot be attributed to electrostatic effects, nor to differences in surface chemistry, but possibly to pure geometrical effects, as increasing the nanowire diameter results in a decreased amount of adsorbed protein. The increased adsorption of laminin on nanowires may explain the exceptionally beneficial properties of nanowire substrates for cellular growth reported in the literature since laminin is often used as surface coating prior to cell cultures in order to promote cell growth, and also because primary cell suspensions contain endogenous laminin.

Identifying crystallization- and incorporation-limited regimes during vapor-liquid-solid growth of Si nanowires.

The vapor-liquid-solid (VLS) mechanism is widely used for the synthesis of semiconductor nanowires (NWs), yet several aspects of the mechanism are not fully understood. Here, we present comprehensive experimental measurements on the growth rate of Au-catalyzed Si NWs over a range of temperatures (365-480 °C), diameters (30-200 nm), and pressures (0.1-1.6 Torr SiH4). We develop a kinetic model of VLS growth that includes (1) Si incorporation into the liquid Au-Si catalyst, (2) Si evaporation from the catalyst surface, and (3) Si crystallization at the catalyst-NW interface. This simple model quantitatively explains growth rate data collected over more than 65 distinct synthetic conditions. Surprisingly, upon increasing the temperature and/or pressure, the analysis reveals an abrupt transition from a diameter-independent growth rate that is limited by incorporation to a diameter-dependent growth rate that is limited by crystallization. The identification of two distinct growth regimes provides insight into the synthetic conditions needed for specific NW-based technologies, and our kinetic model provides a straightforward framework for understanding VLS growth with a range of metal catalysts and semiconductor materials.

Observation and Control of Interfacial Defects in ZnO/ZnSe Coaxial Nanowires

Observation and Control of Interfacial Defects in ZnO/ZnSe Coaxial Nanowires W.A. Bhutto, Z.M. Wu∗, Y.Y. Cao, W.P. Wang and J.Y. Kang∗ Fujian Key Laboratory of Semiconductor Materials and Applications, Department of Physics, Xiamen University Xiamen 361005, P.R. China ZnO/ZnSe coaxial nanowires with di erent ZnO core diameters were synthesized by using a two-step chemical vapor deposition. The scanning electron microscopy images demonstrated that the coaxial nanowires with small ZnO core diameter had the smoother surface than that with large ZnO core diameter. A coherent ZnSe layer with wurtzite structure was observed in the nanowire interface between the ZnO core and the ZnSe shell by high resolution transmission electron microscopy. This coherent layer is bene cial to reduce the defect density and improve the crystal quality by suppressing the phase transition. It was found that the coherent thickness was signi cantly related to the ZnO core diameter. For the nanowire with large ZnO core, a thin critical thickness of 2 3 nm was obtained. As a result, a layer of zinc blende ZnSe appeared outside the nanowire, and a lot of defects existed in the interface between the ZnSe layers with di erent phase structures. For the nanowire with small ZnO core, however, the critical thickness increased and a coherent coaxial structure was observed with the same lattice spacing in the ZnO core and the ZnSe shell. To obtain defect-free coaxial nanowire, an optimal structure was also proposed by theoretical calculation.

CdSe/ZnS Quantum Dot-to-ZnO Nanowires Charge Transfer Dynamics for Enhanced Efficiency Quantum Dot-Sensitized Solar Cells

ABSTRACTCore-shell quantum dots (QDs) with enhanced photostability compared with bare QDs are promising light absorbers for solar cell applications. In this work, electron injection from excited CdSe/ZnS QDs to Zinc Oxide (ZnO) nanowires (NWs) prepared by two techniques were demonstrated. Arrays of ZnO NWs were fabricated by hydrothermal growth and etching. ZnO NWs were sensitized with hydrophobically ligated colloidal CdSe/ZnS QDs. The electron transfer dynamic in QD/ZnO NW architecture was examined using photoluminescence (PL) and decay lifetime analyses. The quenching of the QD emission peak and lowered average lifetime in QD/ZnO NW architecture confirms the deactivation of the excited QDs via electron transfer to ZnO NWs. Electron transfer was enhanced by using smaller QDs. This study provides insight on charge transfer dynamics at the QD/ZnO NW interface in order to engineer high performance quantum dot sensitized solar cells (QDSSCs).

Ni silicide nanowires analysis by atom probe tomography

The Ni silicide formed at low temperature on Si nanowire has been analyzed by atom probe tomography (APT) thanks to a special technique for sample preparation. A method of preparation has been developed using the focused ion beam (FIB) for the APT analysis of nanowires (NWs). This method allow for the measurement of the radial distribution when a NW is cut, buried in a protective metal matrix, and finally mounted on the APT support post. This method was used for phosphorous doped Si NWs with or without a silicide shell, and allows obtaining the concentration and distribution of chemical elements in three-dimensions (3D) in the radial direction of the NWs. The distribution of atoms in the NWs has been measured including dopants and Au contamination. These measurements show that δ-Ni2Si phase is formed on Si NW, Au is found as cluster at the Ni/δ-Ni2Si interface and P is segregated at the δ-Ni2Si/ Si NW interface. The results obtained on NWs after silicidation were compared with the silicide on the Si substrate, showing that the same silicide phase δ-Ni2Si formed in both cases (NWs and substrate).

The Effect of Solvatochromism on the Interfacial Morphology of P3HT-CdS Nanowire Nanohybrids

Increasing performance in organic/inorganic bulk heterojunctions hybrid photovoltaic systems hinges not only on the structure of the inorganic component but also on the morphology of the polymeric component. Changing the morphology of the organic component is a facile way of changing the morphology of the interface between the inorganic and organic components in the bulk heterojunction system. Engineering this interface to more efficiently split photogenerated excitons and transport these carriers to electrodes can increase the efficiency of photovoltaic devices. In this report, we investigate the effect of solvent quality on the morphology of the poly(3-hexylthiophene)-2,5-diyl (P3HT) polymer-semiconductor interface (solvatochromism). We have found that creating more order within the P3HT main chain in solution and prior to deposition has a profound effect on the nature of the P3HT-CdS nanowire interface. Solvents with a larger difference in solubility parameter, Δδ, relative to P3HT, such as methanol and isopropanol, create larger rod domains in the P3HT main chain and result in larger domains of crystalline P3HT at the interface. Solvents with similar solubility parameters as P3HT, such as pyridine and hexanol, create relatively shorter rod domains in the main chain and, as a result, nanohybrids with reduced crystallinity. The results of this paper further cement the importance of manipulating the rod-coil transition in the conducting polymers such as P3HT to improve the crystallinity at the polymer-semiconductor interface that can easily be scaled up to improve the efficiency of bulk heterojunction photovoltatic systems.

Atomistics of vapour-liquid-solid nanowire growth.

Vapour–liquid–solid route and its variants are routinely used for scalable synthesis of semiconducting nanowires, yet the fundamental growth processes remain unknown. Here we employ atomic-scale computations based on model potentials to study the stability and growth of gold-catalysed silicon nanowires. Equilibrium studies uncover segregation at the solid-like surface of the catalyst particle, a liquid AuSi droplet, and a silicon-rich droplet–nanowire interface enveloped by heterogeneous truncating facets. Supersaturation of the droplets leads to rapid one-dimensional growth on the truncating facets and much slower nucleation-controlled two-dimensional growth on the main facet. Surface diffusion is suppressed and the excess Si flux occurs through the droplet bulk which, together with the Si-rich interface and contact line, lowers the nucleation barrier on the main facet. The ensuing step flow is modified by Au diffusion away from the step edges. Our study highlights key interfacial characteristics for morphological and compositional control of semiconducting nanowire arrays.

Stress redistribution in individual ultrathin strained silicon nanowires: a high-resolution polarized Raman study

Strain nano-engineering provides valuable opportunities to create high-performance nanodevices by a precise tailoring of semiconductor band structure. Achieving these enhanced capabilities has sparked a surge of interest in controlling strain on the nanoscale. In this work, the stress behavior in ultrathin strained silicon nanowires directly on oxide is elucidated using background-free, high-resolution polarized Raman spectroscopy. We established a theoretical framework to quantify the stress from Raman shifts taking into account the anisotropy associated with the nanowire quasi-one-dimensional morphology. The investigated nanowires have lateral dimensions of 30, 50 and 80 nm and a length of 1 μm top-down fabricated by patterning and etching 15 nm thick biaxially tensile strained silicon nanomembranes generated using heteroepitaxy and ultrathin layer transfer. The concern over the contribution of Raman scattering at the nanowire 〈110〉 oriented sidewalls is circumvented by precisely selecting the incident polarization relative to the sidewalls of the nanowire, thus enabling an accurate and rigorous analysis of stress profiles in individual nanowires. Unlike suspended nanowires, which become uniaxially strained as a result of free surface-induced relaxation, we demonstrated that stress profiles in single nanowires are rather complex and non-uniform along different directions due to the oxide–nanowire interface. As a general trend, higher stresses are observed at the center of the nanowire and found to decrease linearly as a function of the nanowire width. Using multi-wavelength high-resolution Raman spectroscopy, we also extracted the stress profiles at different depths in the nanowire. The residual stress in the top ∼10 nm of the nanowire was found to be nearly uniaxial and increase from the edge toward the center, which remains highly strained. In contrast, the average stress profiles measured over the whole nanowire thickness exhibit different behavior characterized by a plateau in the region ∼200 nm away from the edges. Our observations indicate that the lattice near the newly formed free surface moves inwards and drags the underlying substrate leading to a complex redistribution of stress. This nanoscale patterning-induced relaxation has direct implications for electrical and mechanical properties of strained silicon nanowires and provides myriad opportunities to create entirely new strained-engineered nanoscale devices.

Photoelectric probing of the interfacial trap density-of-states in ZnO nanowire field-effect transistors

We have fabricated transparent top-gate ZnO nanowire (NW) field effect transistors (FETs) on glass and measured their trap density-of-states (DOS) at the dielectric/ZnO NW interface with monochromatic photon beams during their operation. Our photon-probe method showed clear signatures of charge trap DOS at the interface, located near 2.3, 2.7, and 2.9 eV below the conduction band edge. The DOS information was utilized for the photo-detecting application of our transparent NW-FETs, which demonstrated fast and sensitive photo-detection of visible lights.